SOLUTION: Please help me solve this question: Paul is born on his grandfather's birthday. In how many years will his grandfather be twice as old as Paul?

If his grandfather turned G years old on his birthday, then in G more years Paul's grandfather will be twice as old:



Now: 

Paul is 0

Grandfather is G



G years into future:

Paul will be G 

Grandfather will be G+G = 2G  (= twice Paul's age)